Conventional multi-beam communications satellites (e.g., spot beam medium earth orbit (MEO) and low earth orbit (LEO) satellites) and high altitude platforms (HAPs) are generally designed in a manner whereby a given geographic coverage area is serviced by a pattern of beams defined based on the geometry of the antenna (e.g., employing conventional parabolic reflector antennas). In such conventional designs, the radiation patterns of the satellite/HAP antennae are fixed and consequently generate a configuration of beams that are fixed in terms of their scan angle, beam width and directivity as preconfigured on the satellite/HAP. These beams sweep across the ground as the satellite/HAP moves though its orbit or flight pattern. A user terminal at a fixed geographical location is thus served sequentially by the different beams as the fixed beams sweep across the geographic site or cell where the terminal is located. For example, in a satellite or HAP system, the period of a satellite orbit is determined by the altitude of the satellite orbit. For example, for a typical LEO satellite at an orbit of about 1200 km altitude, the orbital period is less than two hours. Further, since such a system typically employs beams with high directivity corresponding to small beam widths, the user terminal experiences a beam handover as often as every 10 to 20 seconds, where each handover may require a change in frequency and polarization on the part of the terminal, which has to be synchronized at the satellite.
Accordingly, the processing for each handover requires frequent, and as such inefficient, use of the computing and transmission resources of the satellite/HAP. Further, from the time that a handover from one beam to another becomes necessary and the time the handover is completed, the communications link with the current beam may be lost or degraded, and thus the data transmissions are interrupted until the link with the new beam is established. In such cases the data transmissions must be terminated prior to the point of the loss of transmission capability and until the transmissions can be resumed via the new beam, otherwise the respective data packets may be lost and have to be retransmitted. Then, once the transmissions are resumed, the system must “catch up” with the data transmissions and first transmit the delayed packets—however, this introduces latency in data transmission as well as latency jitter, which may be unacceptable for certain applications (e.g., real-time applications such as voice over IP).
For example, FIG. 1 illustrates a cell pattern on the ground as would be illuminated by a conventional multi beam reflector antenna with a feed array or a phased array antenna, generating a beam pattern fixed with respect to the satellite/HAP frame of reference. The cell pattern of FIG. 1 illustrates the projected beam pattern based on a LEO satellite, at a given altitude, which is fitted with an antenna structure (for example a reflector antenna or a phased array antenna) that forms fixed 3.22 degree diameter beams in any direction. As is apparent from the illustrated example, while all of the beams are identical from the perspective of the satellite (e.g., forming 3.22 degree beams in the azimuth and elevation sphere of the satellite), as projected onto the ground, the beams vary in size from 70 km in diameter to 560 km diameter (along the longest diameter of the oblong-shaped cells). Accordingly, the bit rate density as measured in bits per second per kilometer squared will vary considerably. Also, each beam is preconfigured to operate at a fixed frequency/polarization resource. Hence, as the beams move over a ground terminal, the terminal must be handed-over from beam-to-beam as the beam cells move across the surface of the Earth where the terminal is situated. This requires the terminal to continually adjust its frequency/polarization to match that of the satellite beam within which it is currently located.
What is needed, therefore, are approaches for wireless communications systems (e.g., satellite communications systems) that employ spot-beam (or cell) patterns that are fixed with respect to the earth frame of reference (i.e., fixed ground-based beams).